The invention relates to a method for operating a laser system and to a laser system for bleaching teeth.
White teeth have long been considered cosmetically desirable. For this reason, methods have been introduced to whiten teeth that are naturally off-white or have become stained by smoking or food intake. Teeth whitening can be achieved in two ways: firstly, with gels, pastes or liquids including toothpastes that are mechanically agitated at the stained tooth surface in order to effect tooth stain removal through abrasive erosion of the stained acquired pellicle; and, secondly, with gels, pastes or liquids that accomplish the tooth bleaching effect by a chemical process while in contact with the stained tooth surface for a specified period of time, after which the formulation is removed. The present invention pertains to chemical strategies for removing or destroying tooth stains by means of aqueous gels or pastes.
Bleaching gels typically consist of water and at least one bleaching agent selected from the group consisting of hydrogen peroxide and compounds that release hydrogen peroxide (H2O2) in water. In addition, gels contain also some or all of the following compounds: a thickening agent (for example, polyacrylic acid); a stabilizing agent (for example, amino carboxylic acid/salt); and a neutralizing agent that serves to neutralize the thickening agent. The gels are used in that the aqueous gel is applied to the teeth and removed after a certain residence time. Water is the principal component of the aqueous gel and is preferably present in an amount of more than 50% by weight. The bleaching agent is present in an amount ranging from 3% to 50%, most preferably 35%, by weight of the aqueous gel. Higher amounts of bleaching agent are preferred so that the gel may serve as a “fast acting bleaching gel” capable of bleaching teeth with only one or two applications. The right combination of thickening agent and stabilizing agent provides a gel that can be loaded with hydrogen peroxide and remains sufficiently stable to provide a suitable gel in regard to viscosity considerations and with little hydrogen peroxide decomposition after four to twelve weeks storage at room temperature and even longer periods of time when refrigerated.
In a typical treatment process, the dental bleaching gel is brought into contact with the teeth that are to be bleached. The dental bleach is then allowed to remain in contact with the teeth for a residence time ranging anywhere from 5 minutes to one hour. The degree of whitening provided by the gel increases with time of contact between the reactive species of peroxide and the tooth enamel surface, and the rate of activation of the gel in terms of generating available peroxide (H2O2) and/or its reactive species (OH and O). Due to its chemical structure, the peroxide must produce transient species, such as OH and O, before the final products, H2O and O2 are generated by the following mechanism:2H2O2=4OH=2H2O+2O=2H2O+O2 
The presence of the active transient species (radical OH and atomic oxygen O) plays the most important role in the whitening process due to the high reactivity of the transient species.
The bleaching effect is directly proportional to the residence time. However, the bleaching effect can be amplified by applying a heating element, a heat light, or a laser light to the dental bleaching gel once it is in place on the teeth. The heat and light serve to increase the rate of bleaching of the hydrogen peroxide, providing a shorter period of time in which whitening of the teeth is achieved. The increased effectiveness and speed of the whitening process at higher temperatures is due to the faster generation and mobility of H2O2 in the peroxide gel, the decomposition of H2O2 to OH and O, the enhanced diffusion rate into the tooth as well as the enhanced reaction time between the active peroxide species (which can be radicals of OH or atomic oxygen O) and the compounds of the enamel and dentin. Typical temperature increases that are desirable in such procedures are between 10° C. and 30° C.
When laser light is used for the amplification of the bleaching effect, typically a plurality of discrete laser enhancing particles are added to the gel. The particles are capable of absorbing the light energy from the wavelength of light emitted from the laser and of re-transmitting the light energy as thermal energy. These particles are dispersed throughout the bleaching compositions so that the laser beam can pass through the surface of the tooth while the particles absorb a portion of the light energy from the laser and retransmit it as thermal energy thus increasing the effectiveness of the bleaching composition. For example, an argon laser utilizes a blue light with a wavelength in the range of 470 nm to 520 nm. The complementary color to blue is orange, and thus an orange or red-colored or pigmented particulate material that absorbs in this range would be suitable. Also preferred are other colors that absorb at the wavelength of the utilized laser light. For example, a black particulate material absorbs across all wavelengths and would thus also be suitable. Other typically used lasers for heat enhanced teeth whitening are diode lasers with a wavelength of 810 nm or Nd:YAG lasers with a wavelength of 1,064 nm.
Some of the disadvantages of the existing laser light enhancing methods will be explained in the following. A special bleaching gel that contains additional laser light absorbing particles must be used. A special laser device that emits at the absorbing wavelength of the enhancing particles must be used. Special care must be taken that the laser enhancing particles are non-poisonous and biocompatible. It may be difficult to clean the teeth colored by the laser enhancing particles after the procedure. The density of the added absorbing particles is typically such that the laser light is not fully absorbed in the relatively thin layer of the gel that is deposited on the tooth surface. As a result, the laser is transmitted to the tooth surface and possibly transmitted into the dental tissue. This can result in an undesirable heating of the hard dental tissue and of the dental pulp, possibly leading to pain and irreversible damage. Indeed, some of the treatment procedures recommend applying laser light to a tooth for a duration within a certain range until the patient reports feeling pain.
In a completely different scenario, far infrared lasers are used for ablative applications in dentistry. At certain wavelengths of the laser a high absorption in water is encountered causing heating and vaporization of the water. Since human tissue contains at least some water, the laser energy absorption in water is used for causing sudden vaporization similar to micro-explosions which leads to tissue ablation. However, such far infrared laser heating has not been proposed for teeth whitening for numerous reasons.
Firstly, there is a significant safety concern. As water is an important component of hard dental tissues, enamel and dentin as well, these wavelengths are therefore highly absorbed in the enamel and dentin, and for this reason are used in hard tissue treatments, such as cavity preparations or enamel etching. There is therefore a danger that unnecessary damage to the enamel might occur when the radiation from these lasers is accidentally directed to an unprotected area of the tooth during a teeth whitening procedure.
Secondly, there is also an efficacy concern. Because of the extremely high absorption in water, these laser wavelengths are absorbed in a very shallow, only approximately three micron deep, surface layer of the aqueous bleaching gel. As a result, the surface of the bleaching gel can become very hot and can even be vaporized while the deeper layers of the gel are not affected and not heated by the laser light. These problems are particularly pronounced due to the fact that most of the known far infrared dental ablative lasers operate effectively only in a pulsed mode with relatively short laser pulses and relatively high laser pulse power. Far infrared ablative lasers have therefore not been proposed for use in teeth whitening.